How can I reverse the order of lines in a file in Linux?

To reverse the order of lines in a file in Linux, you can use several command-line tools. Below are multiple methods with detailed explanations and examples:

1. Using tac (Reverse cat)

The simplest and most efficient method is the tac command, which reverses lines in a file.

Syntax:

tac [input_file] > [output_file]

Example:

# Create a sample file
echo -e "Line 1\nLine 2\nLine 3\nLine 4" > input.txt

# Reverse lines and save to output.txt
tac input.txt > reversed.txt

# View the result
cat reversed.txt

Output:

Line 4
Line 3
Line 2
Line 1

2. Using awk

Use awk to store lines in an array and print them in reverse order.

Syntax:

awk '{ lines[NR] = $0 } END { for (i=NR; i>=1; i--) print lines[i] }' [input_file]

Example:

awk '{ lines[NR] = $0 } END { for (i=NR; i>=1; i--) print lines[i] }' input.txt > reversed.txt

Output (same as tac):

Line 4
Line 3
Line 2
Line 1

3. Using sed

A less common but creative approach with sed.

Syntax:

sed '1!G;h;$!d' [input_file]

Example:

sed '1!G;h;$!d' input.txt > reversed.txt

How It Works:

• 1!G: Append lines (except the first) to the hold space.
• h: Overwrite the hold space with the current line.
• $!d: Delete all lines except the last, which is printed.

4. Using nl, sort, and cut

Use line numbering and reverse sorting.

Syntax:

nl -ba -nrz [input_file] | sort -r | cut -f2-

Example:

nl -ba -nrz input.txt | sort -r | cut -f2- > reversed.txt

Explanation:

1. nl -ba -nrz: Number all lines (-ba), including empty ones, with right-justified zero-padded numbers.
2. sort -r: Sort lines in reverse order.
3. cut -f2-: Remove the line numbers.

5. Using perl

A Perl one-liner to reverse lines.

Syntax:

perl -e 'print reverse <>' [input_file]

Example:

perl -e 'print reverse <>' input.txt > reversed.txt

6. Using Python

A Python script to reverse lines.

Script:

python3 -c 'import sys; print("\n".join(reversed(sys.stdin.read().splitlines())),' < input.txt > reversed.txt

Summary of Methods

Method	Command	Use Case
tac	tac input.txt > reversed.txt	Fastest and simplest for most cases.
awk	awk '{ ... }' input.txt	Flexible for scripting.
sed	sed '1!G;h;$!d' input.txt	Compact but less intuitive.
nl/sort	nl ... | sort -r | cut ...	Works without tac.
perl	perl -e 'print reverse <>' input.txt	Perl one-liner.
Python	python3 -c '...' < input.txt	Cross-platform scripting.

Notes:

1. Performance:

• tac is the fastest for large files.
• Methods like awk or perl load the entire file into memory, which may not be suitable for very large files.

1. Edge Cases:

• Empty lines are preserved in the reversed output.
• Special characters (e.g., \n) are handled correctly.

1. Install tac:

• If tac is not available (e.g., on minimal systems), install it via coreutils:
  bash sudo apt-get install coreutils # Debian/Ubuntu

Example Workflow

Reverse a log file and analyze the latest entries first:

tac /var/log/syslog | less